Thermosetting coating composition and its use for primer coatings on metal substrates

ABSTRACT

The present invention relates to a thermosetting coating composition based on a binder composition containing 
     A) prepolymers containing isocyanate groups blocked with ε-caprolactam, having an average molecular weight of 1000 to 10,000 and prepared from 
     A1) aromatic polyisocyanates and 
     A2) a component containing at least one organic compound having hydroxyl groups wherein 50 to 100 hydroxyl equivalent percent of component A2) is based on polyesters containing hydroxyl groups and 0 to 50 hydroxyl equivalent percent is based on polyhydroxyl compounds which are free from ester groups and 
     B) a hardener component containing at least one cycloaliphatic polyamine having at least two primary amino groups, 
     in an equivalent ratio of blocked isocyanate groups to amino groups of 1:1 to 1:1.5. 
     The invention also relates to use of the coating compositions to produce stoving-resistant gravel-proof primer coatings on optionally pretreated metal substrates.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to novel thermosetting coating compositions basedon ε-caprolactam-blocked isocyanate prepolymers and cycloaliphaticpolyamines and use thereof for producing stoving-resistant gravel-proofprimer coatings on metal substrates.

2. Description of the Prior Art

Gravel-proof primer coatings on cars have to meet special mechanicalrequirements-in order to protect the multi-layer coating structure fromdamage from gravel and ultimately to prevent the car-body sheet fromcorrosion.

One known solution is to use coating compositions containingpolyurethane ureas. Polyurethane urea binders for producing thickcoatings must be produced from relatively low molecular components,i.e., blocked NCO prepolymers and diamines. These binders are known anddescribed in the prior art, e.g., in DE-PS 2 131 299. Theε-caprolactam-blocked NCO prepolymers described in this reference areobtained by reacting aromatic diisocyanates with polymerization orpolycondensation products containing hydroxyl groups and are inerttowards amines. The compounds containing hydroxyl groups which are usedto produce the NCO prepolymers are preferably the polyether polyolsknown from polyurethane chemistry. Polyester polyols are not mentionedin this connection, apparently because of the possibility of aminolyticside-reactions between the ester bonds and the free amino groups in theactivator during storage and cross-linking of the coating compositions.The advantage of the system in the citation lies in the interactionbetween hard segments (polyurea groups) and soft segments (polyethercomponents) which provide the viscoplastic coating properties for goodprotection against gravel.

The known binders, however, have the disadvantage of sensitivity to anincrease in the stoving temperature. If they are stoved at exactly 160°C., optimum film properties are obtained. If the stoving temperature isincreased to only 180° C., the film is permanently soft. If the stovingtemperature is 200° C., the film properties become even worse. Theresult, particularly in the "overspray" region, where the thick layer ofanti-gravel coating becomes thin, is often a loss of adhesiveness in thesubsequently-applied filler and additional coating layers.

In practice, particularly in car coatings, the stoving temperaturecannot always be maintained narrowly between 160° and 170° C., asrequired for optimum properties of the known coating compositions. Thereis therefore a demand for "stoving-resistant," coating compositions,which can withstand stoving temperatures of up to 200° C. withoutadverse effects on the properties of the coatings.

Accordingly, it is an object of the present invention to providestoving-resistant coating compositions, particularly coatingcompositions which may used for producing anti-gravel primer coatings.

This objective may be achieved with the coating compositions of thepresent invention which are described in detail hereinafter.

It has unexpectedly been found that coating compositions based onε-caprolactam-blocked NCO prepolymers prepared from aromaticdiisocyanates and polyester polyols in combination with aliphaticpolyamines yield coatings which, in contrast to the coatings of DE-PS 2131 299, give very good results in the anti-gravel test approved by theVerband der Deutschen Automobilindustrie, even under extreme stovingconditions (e.g. 200° C. for 20 minutes). Another surprising finding wasthat the simultaneous presence of ester groups and free amino groups inthe compositions does not result in undesired side reactions during thestoving process or in loss of stability of the coating compositionsduring storage.

SUMMARY OF THE INVENTION

The present invention relates to a thermosetting coating compositionbased on a binder composition containing

A) prepolymers containing isocyanate groups blocked with ε-caprolactam,having an average molecular weight of 1000 to 10,000 and prepared from

A1) aromatic polyisocyanates and

A2) a component containing at least one organic compound having hydroxylgroups wherein 50 to 100 hydroxyl equivalent percent of component A2) isbased on polyesters containing hydroxyl groups and 0 to 50 hydroxylequivalent percent is based on polyhydroxyl compounds which are freefrom ester groups and

B) a hardener component containing at least one cycloaliphatic polyaminehaving at least two primary amino groups,

in an equivalent ratio of blocked isocyanate groups to amino groups of1:1 to 1:1.5.

The invention also relates to use of the coating compositions to producestoving-resistant gravel-proof primer coatings on optionally pretreatedmetal substrates.

DETAILED DESCRIPTION OF THE INVENTION

The aromatic polyisocyanates A1) are preferably commercially availablearomatic polyisocyanates such as 2,4-diisocyanate toluene; mixturesthereof with up to 35 wt. % of 2,6-diisocyanate toluene;4,4'-diisocyanate diphenyl methane; mixtures thereof with up to 50 wt.%, preferably up to 40 wt. % of 2,4'-diisocyanate diphenyl methane, andoptionally up to 5 wt. % of 2,2'-diisocyanate diphenyl methane; mixturesof these diisocyanato diphenyl methane isomers with their higherhomologues containing more than two isocyanate groups, wherein mixturesusually contain at least 80 wt. % of the aforementioned diisocyanatodiphenyl methane isomers; and mixtures of the polyisocyanates mentionedby way of example. In the preceding all references to wt. % are based onthe weight of the mixture.

The polyhydroxyl compounds A2) are predominantly polyesters containinghydroxyl groups or mixtures of polyesters containing hydroxyl groupswith up to 50 hydroxyl equivalent percent of polyhydroxyl compounds freefrom ester groups, preferably polyethers containing hydroxyl groups.These optional polyether polyols may be used to regulate the viscosity.

The polyester polyols, i.e., polyesters containing hydroxyl groups, arepreferably condensation products of polybasic acids or anhydrides ofpolybasic acids with excess quantities of polyhydric alcohols. Suitablepolybasic acids or acid anhydrides include adipic acid, maleic acid,maleic acid anhydride, phthalic acid, phthalic acid anhydride,tetrahydrophthalic acid, tetrahydrophthalic acid anhydride,hexahydrophthalic acid, hexahydrophthalic acid anhydride and mixtures ofthese polybasic acids and/or acid anhydrides.

The polyhydric alcohols include those having a molecular weight of 62 to200 such as ethylene glycol, propylene glycol, the isomeric butanediols,pentanediols, hexanediols and octanediols, glycerol, trimethylol propaneand mixtures of these polyhydric alcohols. The polyester polyols whichare suitable for use as all or a portion of component A2) have amolecular weight of 400 to 4000, preferably 500 to 2000, which can becalculated from the amounts of the starting materials. Particularlypreferred polyester polyols have a molecular weight of 500 to 2000 andare prepared from adipic acid and neopentyl glycol and/or1,6-dihydroxyhexane.

The polyester polyols A2) also include polylactone diols, e.g., reactionproducts of hexanediol-1,6 with ε-capralactone having a molecular weightof 500 to 1500.

Other examples of polyester polyols suitable as all or a portion ofcomponent A2) include polycarbonate diols, preferablyhexanediol-1,6-polycarbonate diols having a molecular weight of 500 to2000, which may be obtained in known manner by condensation of the diolwith diphenyl carbonate or dimethyl carbonate.

A portion of the polyester polyols A2) may be based on oligoesters oflong-chain carboxylic acids containing hydroxyl groups such as castoroil. Use of these esters, however, is less preferable. They are used, ifat all, in quantities not exceeding 30 hydroxyl equivalent of componentA2).

The option polyhydroxy polyethers are known polyaddition products ofepoxides, e.g. ethylene or propylene oxide, to low molecular weightpolyols. It is preferred to use propylene oxide polyethers initiatedwith bisphenol A or trimethylol propane and having a molecular weight of344 to 3000, preferably 344 to 800. The molecular weight is calculatedfrom the content of hydroxyl groups and the functionality. It is alsopossible, though less preferred, to use other polyhydroxyl compoundswhich are free from ester groups as a portion of component A2), e.g.,the polyhydric alcohols having a molecular weight of 62 to 200 whichhave been described as starting materials for the polyester polyols.

The blocked NCO prepolymers may be prepared in known manner, e.g., in atwo-stage reaction by reacting the polyhydroxyl compounds A2) withexcess quantities of polyisocyanates A1. Optionally, a large excess ofthe polyisocyanate may be used followed by removal of unreactedpolyisocyanate after formation of the prepolymer, e.g., by distillation,preferably in a thin layer evaporator. However, the reaction ispreferably conducted while maintaining an NCO/OH equivalent ratio of 2:1to 2.5:1. The nature and proportions of the starting components A1) andA2) are chosen so that the resulting NCO prepolymers have an NCO contentof 3.0 to 7.0, preferably 4.5 to 6.0 wt. %. After preparation of the NCOprepolymers, they are blocked, preferably stoichiometric quantities ofε-caprolactam, optionally in the presence of solvents. Both reactionsare normally carried out at temperatures of 60° to 120° C., preferably80° to 100° C.

Alternatively, the blocked NCO prepolymers A) can be manufactured in aone-pot reaction, wherein the polyhydroxyl compounds A2) and theblocking agent, preferably ε-caprolactam, are reacted with thepolyisocyanate A1) within the aforementioned temperature range until theNCO bands disappear (IR spectrum). As a rough approximation, theproportions of the reactants are chosen so as to provide about onehydroxyl and one equivalent of blocking agent per 2 NCO equivalents.

To obtain coating compositions with optimum spray viscosity, theresulting blocked NCO prepolymers A) can be diluted with about 25 to 50wt. %, based on the weight of the solution, of organic solvents.Examples of suitable solvents include Solvesso 100 solvent, solventnaphtha, isobutanol, butyl acetate and/or methoxypropyl acetate.

The hardener component B) is preferably a cycloaliphatic polyamine withat least two primary amino groups. Examples include4,4'-diaminodicyclohexyl methane, 1,4-diamino cyclohexane,3,3,5-trimethyl-5-aminomethyl cyclohexyl amine (1PDA) and theperhydrogenated triamino diphenyl methanes used in DE-OS 3 417 683 toproduce the corresponding cycloaliphatic triisocyanates. Mixtures ofthese polyamines may also be used.3,3'-dimethyl-4,4'-diamino-dicyclohexyl methane is preferred.

The binder compositions are produced by mixing components A) and B) andoptionally the previously described solvents. The amounts of thecomponents are chosen to provide an equivalent ratio of blocked NCOgroups to amino groups of 1:1 to 1:1.5, preferably 1:1.1 to 1:1.4.

The coating compositions according to the invention may also containconventional coating additives and auxiliaries such as pigments,levelling agents, UV stabilizers, anti-oxidizing agents, fillers andthixotropic agents.

The binder compositions can be combined with the additives andauxiliaries before, during or after components A) and B) have beenmixed.

The coating compositions according to the invention are particularlysuitable for producing stoving coatings on metal substrates which mayoptionally be pretreated, preferably metal substrates used for carmanufacture. The coatings are preferably stoved at temperatures of 160°to 200° C.

In the following examples, all part and percentages are by weight unlessotherwise indicated.

EXAMPLES EXAMPLE 1

This example describes the preparation of a coating compositionaccording to the invention and the properties of the resulting coating.

    ______________________________________                                        1.1 Preparation of the binder composition                                     ______________________________________                                        Prepolymer Ingredients                                                         510 g (0.6 equiv)                                                                          of an adipic acid/hexanediol/neopentyl                                        glycol polyester (weight ratio of                                             hexanediol to neopentyl glycol = 77:36),                                      OH number 66                                                     110 g (0.4 equiv)                                                                          of a polyoxypropylene glycol initiated                                        with bisphenol A, OH number 204                                  183 g (2.1 equiv)                                                                          of 2,4-toluylene diisocyanate                                    124 g (1.1 moles)                                                                          of ε-caprolactam                                         309 g (75%)  of Solvesso 100 solvent                                         1236 g (1.1 equiv of blocked NCO groups)                                      Binder Ingredients                                                             100 g (0.089 equiv)                                                                        of the aforementioned blocked NCO                                             prepolymer                                                       15.0 g (0.126 equiv NH.sub.2)                                                              of 3,3'-dimethyl-4-4'-diamino                                                 dicyclohexyl methane                                            Equiv ratio of blocked NCO/NH.sub.2 = 1:1.4.                                  ______________________________________                                    

Procedure

The polyester, polyether and toluylene diisocyanate were mixed in thequantities set forth above and heated to about 80° to 90° C. withagitation. After the reaction at 90° C. had proceeded for about 21/2hours, the measured NCO content was 5.5% (calculated 5.7%). Portions ofpulverulent ε-caprolactam were added and the mixture was diluted withSolvesso 100 solvent and reacted at 90° C. for a further 5 hours untilno NCO content was detectable (IR spectrum). The blocked NCO prepolymersolution had a viscosity at 23° C. of about 8000 mPas and a blocked NCOequivalent weight of 1130 g.

The soft segment component, i.e., the polyester/polyether segment of theNCO prepolymer contained 60 equivalent % of a polyester containinghydroxyl groups and 40 equivalent % of polyether containing hydroxylgroups. As is apparent from the weights of the starting components, thepolyester was the dominant hydroxyl group-containing component:

82.3 wt. % polyester

17.7 wt. % polyether.

In order to produce the binder, 100parts of the aforementioned blockedNCO prepolymer-solution were mixed with 15 parts of3,3'-dimethyl-4,4'-diamino dicyclohexyl methane.

    ______________________________________                                        1.2 Production of the coating composition                                     ______________________________________                                        100.0 parts                                                                            of the blocked NCO prepolymer from Example 1.1                       15.0 parts                                                                             of 3,3'-dimethyl-4,4'-diamino dicyclohexyl methane                   11.3 parts                                                                             of titanium dioxide (rutile type)                                    1.2 parts                                                                              of iron oxide black                                                  90.5 parts                                                                             of barium sulphate (extender)                                        2.9 parts                                                                              of highly dispersed silicic acid                                     40.4 parts                                                                             of ethoxypropyl acetate/solvent naphtha 150                          261.3 parts                                                                   ______________________________________                                    

The components were dispersed in a dissolver (7000 rpm) for about 30minutes. The resulting anti-gravel primer was usable in this form in theair or airless process at 40 to 50° C. The coating composition wasstored for two months at 50° C. without a change in viscosity.

1.2.1 Production of test sheets

Phosphated car-body sheets were first coated with a commercial cathodeelectrode position coating (German abbreviation KTL) and stoved at 190°C. for 20 minutes (layer thickness 15 to 20 μ). A layer (dry filmthickness 120 μ) of the primer according to the invention was thenapplied. After air drying for 1 minute, 40 μ of a conventionalpolyester-melamine filler was sprayed "wet on wet" without furtherintermediate drying. In the conventional process the coating is stovedat 160° C. for about 30 minutes. To test the stoving resistance, thecoating was stoved for 20 minutes at 200° C. Finally, a 40 μ thick layerof a commercial acrylate stoving top coat (TSA) was applied and stovedat 160° C. for 20 minutes.

1.3 Stoving test

The coated sheets were subjected to the "VDA" (Verband der DeutschenAutomobilindustrie) test in a gravel test device (model 508 by MessrsErichsen). The test details were given under the following headings:multiple impacts with 500 g steel shot, 4 to 5 mm diameter, bombardingpressure 1.5 to 2 bar, handling time about 5 s. The effects of the testwere then shown by a corrosion test, i.e., the salt mist test accordingto ASTM 3117. This test has ten criteria based on a picture tablebetween the characteristic values 0 to 5.

The anti-gravel primer in this example had the best possible testresult, a rating of 0 to 1, i.e., the coating structure remained intact.There were no shiny metal patches on the laminated sheet, indicating nopenetration through the multi-layer coating structure. The sheet wasgiven the mark 1 because of a few small corrosion spots the size of pinpricks.

Example 2 (Comparison Example)

This example describes the construction of a prior art coatingcomposition and the properties of a coating obtained therewith.

    ______________________________________                                        2.1 Preparation of the binder composition                                     ______________________________________                                        Prepolymer Ingredients                                                        1143.0 g (1.0 equiv)                                                                      of a polyether polyol, OH number 49,                                          produced by the propoxylation of a mixture                                    of trimethylol propane and glycerol in a                                      weight ratio of 2.9:1 and subsequent                                          ethoxylation of the propoxylated product                                      (PO:EO weight ratio = 95:5)                                        135.6 g (1.2 moles)                                                                      of ε-caprolactam                                           191.4 g (2.2 equiv)                                                                      of 2,4-toluylene diisocyanate                                     1470.0 g (1.2 equiv of blocked NCO groups)                                    Binder Ingredients                                                            100.0 g (0.098 equiv)                                                                     of blocked NCO prepolymer                                         16.3 g (0.137 equiv)                                                                      of 3,3'-dimethyl-4,4'-diaminodicyclohexyl                                     methane                                                           Equiv ratio of blocked NCO/NH.sub.2 = 1:1.4                                   ______________________________________                                    

Procedure

The anhydrous polyether, ε-caprolactam and toluylene diisocyanate wereweighed and heated to 100° C. with intermittent agitation. The reactionwas continued at this temperature for about 5 hours until no NCO contentwas detectable (IR spectrum). The blocked, solvent-free NCO prepolymerhad a viscosity at 23° C. of about 30,000 mPas and a blocked NCOequivalent weight of 1225 g.

    ______________________________________                                        2.2 Preparation of the coating composition                                    ______________________________________                                        100.0 parts                                                                            of the blocked NCO prepolymer from Example 2.1                        16.3 parts                                                                            of 3,3'-dimethyl-4,4'-diamino dicyclohexyl methane                    11.6 parts                                                                            of titanium dioxide (rutile type)                                     1.3 parts                                                                             of iron oxide black                                                   90.8 parts                                                                            of barium sulphate (extender)                                         3.0 parts                                                                             of highly dispersed silicic acid                                      50.0 parts                                                                            of ethoxypropyl acetate/solvent naphtha 150                          273.0 parts                                                                   ______________________________________                                    

The components were blended into a sprayable anti-gravel primer asdescribed in Example 1.2 and applied to test sheets which had previouslybeen coated with the cathode electrodeposition coating (KTL) describedin Example 1.2.

2.3 Stoving test

The coated sheets were subjected to the gravel test described in Example1.3.

The test result for the thin-layer overspray region was a rating ofabout 4, the third value from the bottom, i.e., the shiny metal or KTLwas exposed over most of the test area. The anti-gravel primer, fillerand top coat were almost completely removed by the shot from the testdevice. This showed that the adhesion of the anti-gravel primer wasPractically destroyed by the stoving process.

EXAMPLE 3

    ______________________________________                                        3.1 Production of the binder composition                                      ______________________________________                                        Prepolymer Ingredients                                                         600 g (0.6 equiv)                                                                          of a hexanediol-1,6 polycarbonate, OH                                         number 56                                                        110 g (0.4 equiv)                                                                          of the polyoxypropylene glycol initiated                                      with bisphenol A glycol from Example                                          1.1                                                              124 g (1.1 moles)                                                                          of ε-caprolactam                                         183 g (1.0 equiv)                                                                          of 2,4-toluylene diisocyanate (TDI)                              550 g (65%)  Solvesso 100 solvent                                            1567 g (1.1 equiv of blocked NCO groups)                                      Binder Ingredients                                                            100 g (0.0638 equiv) of                                                                     the aforementioned blocked NCO                                                prepolymer                                                       10 g (0.084 equiv NH.sub.2)                                                                of 3,3'-dimethyl-4,4'-diamino                                                 dicyclohexyl methane                                            Equiv ratio of blocked NCO/NH.sub.2 = 1:1.3                                   ______________________________________                                    

Procedure

A solution of the polyols, ε-caprolactam and Solvesso 100 solvent wasprepared by heating (about 50° C.) and agitation. The total quantity ofTDI was poured at one time into the solution and reacted at 100° C.After a reaction time of about 4 hours, no NCO content was detectable.The blocked NCO prepolymer solution had a viscosity at 23° C. of about3400 mPa.s and a blocked NCO equivalent weight of 1425 g.

In order to produce the binder, 100 parts of this prepolymer solutionwere mixed with 10 parts of 3,3'-dimethyl-4,4'-diamino dicyclohexylmethane.

    ______________________________________                                        3.2 Production of the coating composition                                     ______________________________________                                        100.0 parts                                                                           of the blocked NCO prepolymer from Example 3.1                         10.0 parts                                                                           of 3,3'-dimethyl-4,4'-diamino dicyclohexyl methane                     11.5 parts                                                                           of titanium dioxide (rutile type)                                      1.2 parts                                                                            of iron oxide black                                                    90.5 parts                                                                           of barium sulphate (extender)                                          3.3 parts                                                                            of highly dispersed silicic acid                                       50.5 parts                                                                           of ethoxypropyl acetate/solvent naphtha 150                           267.0 parts                                                                   ______________________________________                                    

The aforementioned compounds were converted into a sprayable anti-gravelprimer as described in Examples 1.2 and 1.2.1 and applied to sheets fortesting.

3.3 Result of the Stoving Test

The test sheets coated with the aforementioned anti-gravel primer were"over-stoved" as described in Example 1.2.1 and then subjected to theVDA gravel test (see Example 1.3).

The anti-gravel primer in this example, like that in Example 1, had thebest possible rating of 0 to 1, i.e., the entire coating structureconsisting of KTL, the aforementioned primer and the commercial fillerand top coat described in Example 1 remained intact.

EXAMPLE 4

    ______________________________________                                        4.1 Preparation of the binder composition                                     ______________________________________                                        Prepolymer Ingredients                                                          510 g (0.6 equiv)                                                                         of the adipic acid/hexanediol/neopentyl                                       glycol polyester from Example 1                                  102.0 g (0.3 equiv)                                                                        of castor oil                                                     5.8 g (0.1 equiv)                                                                         of hexanediol-1,6                                                125.0 g (1.1 moles)                                                                        of ε-caprolactam                                         183.0 g (2.1 equiv)                                                                        of 2,4-toluylene diisocyanate (TDI)                              310.2 g (75%)                                                                              of Solvesso 100 solvent                                         1236.0 g (1.1 equiv of blocked NCO groups)                                    Binder Ingredients                                                              100 g (0.089 equiv)                                                                       of the aforementioned NCO prepolymer                             13.8 g (0.116 val NH.sub.2)                                                                of 3,3'-dimethyl-4,4'-diamino                                                 dicyclohexyl methane                                            Equiv ratio of blocked NCO/NH.sub.2 = 1:1.3.                                  ______________________________________                                    

Working

The polyester, hexanediol-1,6, castor oil and ε-caprolactam weredissolved with agitation and heating (about 50° C.) in Solvesso 100solvent. The total quantity of TDI was added and the temperature wasraised to about 90° C. After reacting at 90° C. for about 4 hours, nofurther NCO was detectable. The blocked NCO prepolymer solution had aviscosity at 23° C. of about 6000 mPas and a blocked NCO equivalentweight of 1124 g.

To produce the binder, 100 parts of this prepolymer solution were mixedwith 13.8 parts of 3,3'-dimethyl-4,4'-diamino dicyclohexyl methane.

    ______________________________________                                        4.2 Production of the coating agent                                           ______________________________________                                        100.0 parts                                                                            of the blocked NCO prepolymer from Example 4.1                        13.8 parts                                                                            of 3,3'dimethyl-4,4'-diamino dicyclohexyl methane                     11.0 parts                                                                            of titanium dioxide (rutile type)                                     1.0 part                                                                              of iron oxide black                                                   89.5 parts                                                                            of barium sulphate (extender)                                         2.7 parts                                                                             of highly dispersed silicic acid                                      40.3 parts                                                                            of ethoxypropyl acetate/solvent naphtha 150                          258.3 parts                                                                   ______________________________________                                    

The aforementioned components were dispersed in a dissolver as describedin Example 1.2. The resulting anti-gravel primer was usable in this formin the air or airless process at 40° to 50° C.

4.3 Result of the Stoving Test

As described in Example 1.2.1 test sheet were prepared and thensubjected to the VDA gravel test described in Example 1.3.

The test result, as in Example 1, was the best possible, i.e., a ratingof 0 to 1. The coating structure was completely undamaged.

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

What is claimed is:
 1. A thermosetting coating composition containing abinder composition wherein the binder composition consists essentiallyofA) a prepolymer containing isocyanate groups blocked withε-caprolactam, having an average molecular weight of 1000 to 10,000 andprepared fromA1) an aromatic polyisocyanate and A2) a componentcontaining at least one organic compound having hydroxyl groups wherein50 to 100 hydroxyl equivalent percent of component A2) is based on apolyester containing hydroxyl groups and 0 to 50 hydroxyl equivalentpercent is based on a polyhydroxyl compound which is free from estergroups and B) a hardener component containing at least onecycloaliphatic polyamine having at least two primary amino groups, in anequivalent ratio of blocked isocyanate groups to amino groups of 1:1 to1:1.5.
 2. A coated substrate wherein the coating is obtained from athermosetting coating composition containing a binder compositionwherein the binder composition consists essentially ofA) a prepolymercontaining isocyanate groups blocked with ε-caprolactam, having anaverage molecular weight of 1000 to 10,000 and prepared fromA1) anaromatic polyisocyanate and A2) a component containing at least oneorganic compound having hydroxyl groups wherein 50 to 100 hydroxylequivalent percent of component A2) is based on a polyester containinghydroxyl groups and 0 to 50 hydroxyl equivalent percent is based on apolyhydroxyl compound which is free from ester groups and B) a hardenercomponent containing at least one cycloaliphatic polyamine having atleast two primary amino groups, in an equivalent ratio of blockedisocyanate groups to amino groups of 1:1 to 1:1.5.